Servosteering, especially for motor vehicles

ABSTRACT

A servosteering control valve is provided with fluid reaction chambers which are pressued responsive to the speed of a vehicle for effecting a simulated steering resistance wherein at low speed, as in parking, there is no steering resistance and resistance increases with speed via feedback from the chambers of a double acting servomotor. The novel feature of the arrangement herein utilizes a coaction between the inlet flow control passage and return flow control passage of a steering control spool or piston valve, one such valve for each chamber. This is in conjunction with throttle passages disposed between the manually shiftable piston valves and a valve housing. Such passages are created by portions of the shiftable valves having elements slightly smaller in diameter than the diameter of the housing bore or bores in which the valves are shiftable. A single bore is used where the valves are coaxially integral and respective bores are used where the valves are separably movable. The arrangement places the throttle passages in flow series between the servomotor chambers and the reaction chambers. The effect of the throttle passages and their relationship with the inlet and return flow control passages results in pressure control of the reaction chambers such that at low speeds the simulated steering resistance is low or non-existent, as in parking, but varying to higher resistance at higher speeds.

BACKGROUND OF THE INVENTION

It has been known basically to vary the simulated resistance to manualsteering effort by way of increased resistance in response to increasein vehicle speed. For example, the U.S. patent to Uchiyama et al, U.S.Pat. No. 3,690,400 shows such an arrangement and other patents in thefield are German DE-PS No. 21 27 070 and German DE-GM No. 80 29 580.

In particular, the patents or applications of Armin Lang, assigned tothe present assignee, viz., German DE No. 31 22 370 (application), whichbecame the present U.S. Pat. No. 4,438,827 and U.S. Pat. No. 4,462,478,are of particular pertinence as background art in that they showarrangements utilizing control valve means such as integral coaxialpiston valves and separately movable piston valves, including fixedthrottles in lines from the chambers of a double acting servomotor toreaction chamber of the valves for exerting pressure forces in oppositedirections.

U.S. Pat. No. 4,438,827 is relied on for the closest background of thepresent invention and is hereby incorporated by reference along withU.S. Pat. No. 4,186,818 of Jablonsky, likewise assigned to the presentassignee.

Thus, in the disclosure herein it will be understood by persons familiarwith the art that the mechanical force which returns the reciprocalvalve means to a neutral position after a steering operation is effectedby springs such as 62, 62A, as shown in U.S. Pat. No. 4,186,818, FIG. 3.Also, the arrangement of a rotary valve housing which carries thereciprocal valves, as used in most of the modifications herein is shownin that patent together with conventional structure as shown to completean operative booster steering device familiar to persons skilled in theart.

BRIEF DESCRIPTION OF THE INVENTION

All modifications use a manually shifted control valve of the reciprocalspool or piston type for each chamber of a double acting servomotor. Inone modification the valves are in a fixed housing and in othermodifications the valves are in a rotative housing of a heretofore knowntype as shown in U.S. Pat. No. 4,186,818 heretofore mentioned, and forthe basic construction, it is shown and described in U.S. Pat. No.4,438,827.

The latter patent shows arrangements of control valve means, one suchvalve mean for each chamber of a double acting servomotor. Thus, thevalve means being in the form of piston valves, spaced collars areprovided which coact with edges of grooves in housing bore means inwhich the piston valves are shiftable by manual effort on a steeringwheel.

There are flow control gaps between collar edges and groove edges whichare open in neutral position whereby the output of a servopump flowsthrough to a tank and to the chambers of a servomotor. Thus, each pistonvalve has an inlet flow control gap and a return flow control gap and areaction chamber.

The servomotor has a housing passage for each chamber and each housingpassage is located between the inlet and return flow control gaps of arespective piston valve.

Dependent upon a steering operation and via a speed responsive variablethrottle, connecting from a tank through fixed throttles to the reactionchambers, pressures in the housing can open check valves leading to thereaction chambers for producing a differential of pressure therein forsimulated steering resistance.

As a novel feature of the present invention a balanced force during slowspeed parking is realized so that the vehicle driver need not workagainst simulated steering resistance. However, upon increase of speedand especially at high speeds suitable steering resistance is effectedas a safety measure.

Accordingly, the advantage of ease of parking effort but increase ofsteering resistance with speed is achieved. Further, the inventionprovides for simulated steering resistance occurring at higher speedsbefore booster support occurs. Thus the mechanical valve centeringforces can be kept low in order to keep parking effort low, but safetyis assured by an equivalent of a high mechanical centering force byvirtue of the feedback to the reaction chambers in advance of powersteering effect on the steering mechanism. Accordingly, there is no needto use a strong mechanical centering spring device for travel safety infast straight ahead travel. The operating characteristics of theinvention eliminate the need for providing a strong centering forcewhich must be overcome by driver effort at the steering wheel.

In particular, although steering effort for parking must always be atleast equal to the mechanical centering force, in the case of steeringcorrections in straight ahead fast travel, only small road reactionforces will occur at the steered wheel of the vehicle to be transmittedto the steering wheel. This can occur only if a strong mechanical valverestoring force to neutral is present, which means strong returnsprings, unfavorable for parking ease.

The invention provides reaction chamber pressure to increase thesteering resistance, avoiding the need for strong centering springforces in making steering corrections during fast straight ahead travel,therefore guarding against oversteering. Further, such similatedsteering resistance occurs without power boost of the servomotor.

The improved results are brought about by providing unequally open flowgaps for inlet and return flow control of each piston valve and byfurther providing the piston valves with throttle means such as collarspredeterminedly spaced from the housing bores so to form throttle gapsto throttle flow from respective chambers of the servomotor to thereaction chambers. Thus throttle gaps are located between a respectiveservomotor passage in the housing and respective ports in the pistonvalve communicating with the reaction chambers.

A detailed description of the invention now follows in conjunction withthe appended drawing in which:

FIG. 1 is a graph of characteristic lines showing the functions of priorart devices wherein Δp represents differential pressures of theservomotor chambers, Md represents the manual torque on the steeringwheel and Mx₁ is mechanical force acting on the control piston valve, orvalves, for restoring neutral position in straight ahead steering, viz.,spring force.

FIG. 2 has the same symbols for the same functions of the inventionwherein Mx₂ represents the mechanical restoring, return spring force,and the added symbol Hx₂ represents hydraulic centering force due toreaction chamber pressure difference.

FIG. 3 shows one embodiment, in cross section, of the invention inneutral position utilizing integral piston valves operable by a manualsteering wheel;

FIGS. 4 through 7 show variations in cross section of the invention inneutral position, all of which use a pair of piston valves.

In the modifications, in order to avoid complexity of referencenumerals, the same reference numerals have been used for similarcomponents even though differing slightly in arrangement.

Referring to FIG. 3, a steering booster pressure control valve 1 isshown understood to be disposed in the steering gear of a vehicle (notshown) and being in neutral position wherein a booster pump output isbypassed to a tank. A steering control piston valve 2 is slidable in avalve housing 3, being actually two integral coaxial piston valves, forconvenience referred to as a piston valve, obviously a pair of valvesshiftable by a manually operative steering wheel, shown in phantomlines, all as depicted in FIG. 3, together with symbolic elements of thehydraulic circuitry, such as an engine driven servopump or booster pump,an oil tank, and a double acting servo motor. Oil under pressure is fedto the control valve 1 by the servopump 4 via an inlet connection 5which branches, as shown, to a pair of annular grooves 6 and 7 in thehousing. A pair of grooves 8 and 9 are provided in the valve piston 2and communicate via lines 10, 11 and 12, 13 with the respective chambersof servomotor 14. A central annular groove 15 in the housing has areturn connection 16 to a return line 17 for return flow to a tank 18.

Four flow control edges are effected by the annular grooves 6 and 7 ofthe housing and the respective grooves 8 and 9 of the piston valvewhereby two inlet flow control gaps 19 and 20 are formed to control flowto the respective servomotor chambers. A central annular groove isprovided in the housing with a valve collar 21 centrally located thereinin neutral position. Thus, pairs of return flow control gaps 22 and 23are effected for controlling return flow from the respective chambers ofthe servomotor.

In FIG. 3, as in all modifications, details of the spring return forceare not shown in order to avoid complexity in the drawing. Anyconventional mechanical return device is usable, e.g., as heretoforeknown in U.S. Pat. No. 4,186,818, having springs.

The ends of piston valve 2 have housing reaction chambers 24 and 25which connect via respective reaction pressure passages, ducts 26 and27, bored axially in the piston valve body, at the ends thereof.Respective radial bores 28 and 29 connect to ducts 26 and 27. The bores28 and 29 effect ports exposed, respectively, between the inlet andreturn flow control gaps 19 and 22 and the inlet and return flow controlgaps 20 and 23, as shown. Spring biased check valves 30 and 31 controlflow from the reaction chamber flow ducts 26 and 27 into respectivereaction chambers 24 and 25. Thus, ducts feed from locations between theinlet and return flow gaps 19-22, 20-23, respectively, into respectivereaction chambers when pressure effects opening of respective checkvalves.

The ports are indicated in FIG. 3 by the ends of the lead lines fromreference numerals 28, 29 and will be easily recognized in the otherfigures of the drawing.

The pressure in the reaction chambers 24 and 25 are controlled viarespective lines 32 and 33 connecting to a passage 35 at a junction 34.The lines 32 and 33 for reaction chambers 24 and 25 connecting tojunction 34 are provided with respective fixed throttling orifices 36and 37. Passage 35 connects via a speed responsive throttle, such as avariable orifice 38, to the tank 18.

Variable speed responsive arrangements are known for the control orifice38, for example, in German Pat. No. 21 27 070 as well as in U.S. Pat.No. 3,690,400 showing a nozzle-baffle plate system, and in Germanutility patent 80 29 580 showing a solenoid operated device. Inparticular, the U.S. patent describes a speed sensing system for thepurpose of reaction chamber pressure control.

Thus, speed responsive variable orifice 38 is shown in line 35 betweenreaction chambers 24 and 25 and tank 18 wherein the orifice crosssection is automatically adjusted for throttling effect in dependence onthe speed of travel of the vehicle. However, the variable throttle 38with connection to tank 18 could also be, e.g., a variable throttle 38in the inlet line of booster pump 4. In that case, there would be someadvantage in the construction and dimension of the chambers 24 and 25.Such a change is not essential to the invention and need not bedescribed in detail. In any event such arrangement is shown anddescribed in U.S. Pat. No. 4,438,827.

The connecting lines 10 and 11 to respective sides of the servomotor 14and to respective ports 28 and 29 of the reaction chamber ducts 26 and27 to reaction chambers 24 and 25 are provided with respectivethrottling passages 39 and 40. Such throttling passages 39 and 40 areeffected between a respective narrow throttling collar 41 and 42, on thepiston valve, and the surface of the bore 43 in the housing in which thepiston valve 2 is slidable. Thus, as will be seen in FIG. 3, thediameters of the collars are slightly smaller than the piston valveends, which equal the central collar. The throttling collars, shiftableaxially with respect to the bore, cause piston valve grooves 8 and 9 tobe divided into partial grooves 8A, 8B and 9A, 9B which vary in axiallength independent on the piston valve position.

The inlet pressure flow control gaps 19 and 20 and the return flowreturn gaps 22 and 23 are shown with spaced edges, i.e., the gaps 19, 20and 22, 23 are open in neutral position of the piston valve 1. It shouldbe noted that the inlet pressure flow control gaps 19 and 20 are largerthan the respective return flow gaps 22 and 23 in neutral position.

Also, the ports for bores 28, 29 receive flow from the respectiveservomotor passages 10, 11 through respective throttle passage meanssuch as passages or gaps 39, 40. Such flow relationship is true for allmodifications, flow being via the housing from passages 10, 11 to theports as will be apparent.

FIG. 4 is a modification having a rotative housing 44 with a pair ofpiston valves 45 and 46. The piston valves are shifted in oppositedirections by a fork shaped end of a steering spindle (not shown) havingthe spaced actuating FIGS. 47 and 48 in slots of the respective pistons,as shown, to permit the spindle to rotate while the piston valvesreciprocate. This modification is otherwise similarly operative to thatshown in FIG. 3. The piston valves are identical and the symmetricalarrangement shown permits each to effect the functions of the dualpiston valve 2, piston valves 45 and 46 acting in unison. Thus, when onepiston valve is pressurizing a chamber of the servomotor 14, the otherpiston valve is effecting return flow from the other chamber, the pistonvalves shifting in opposite directions when the steering wheel ismanually rotated since the actuating fingers are on opposite sides ofthe center of symmetry. The piston valves 45 and 46 have reactionchambers 24 and 25 respectively at corresponding ends. The inletpressure control gap 19, line 10 for servomotor 14, bore 28 for reactionchamber 24 and return flow gap 22 are disposed on piston valve 45.Throttling collar 41 (similar to FIG. 3) is carried on the piston valveto provide throttling passage 39. Correspondingly, piston valve 46 hasinlet flow control gap 20, line 11 for servomotor 14, bore 29 forreaction chamber 25 and return flow gap 23, with throttling collar 42effecting throttling passage 40 intermediate the inlet and return flowgaps. Further, the fixed throttles 36 and 37 and speed responsivethrottle 39 of the reaction chamber arrangement are as found in FIG. 3.

FIG. 5 shows an arrangement wherein the narrow collars 41 and 42 ofFIGS. 3 and 4 have been replaced by axially elongated collars, 51 and52, stepped portions of the piston valves slightly reduced in diameteras compared with the diameters of piston valves 49 and 50, respectively,which are the diameters of their housing bores. Collars 51 and 52 aredimensioned relative respective housing bores 53 and 54 to effectthrottling passages 55 and 56. The arrangement permits a shorter designof the piston valves since bores 28 and 29 for respective reactionchambers 24 and 25 can then be almost in the same plane axially relativeto piston valves 49 and 50 as the connecting passages thereto for thelines 10 and 11 to servomotor 14. This is possible when bores 28 and 29and those passages for lines 10 and 11 have angularly related axes,e.g., 90° as shown in FIG. 5.

FIG. 6 shows the narrow throttling collars 41 and 24 which are effectiveonly when respective valve pistons 45 and 46 are shifted slightly fromneutral position. The passages for lines 10 and 11 for servomotor 14 areprovided with respective annular grooves 57 and 58 within the respectivehousing bores.

It will be noted that collars 41, 42 are offset from the respectiveedges of grooves 57, 58 to permit a free flow of circulating oil throughopen gap G in neutral position. However, upon slight shift of the pistonvalves 45, 46, the collars being slightly smaller in diameter than therespective housing bores 53 and 54, effect respective throttle passages55, 56 when inserted into the annular boares.

The free flow openings as seen in FIG. 6 are slightly smaller than theinlet flow control gaps 19, 20 and the return flow control gaps 22, 23so that the throttling passages become effective when the piston valvesare shifted in order to coact with the inlet and return gaps.

Thus, upon shift, dependent on direction, throttle passages 59 and 60become effective, then return control flow gap 23 or 22 closes, theninlet flow control gap 19 or 20 closes.

OPERATION (FIGS. 3-7)

Referring to FIGS. 3 and 4, if variable throttle 38 is closed and thepiston valves are in neutral position, flow passes through the housing 3from servopump to tank and the pressure drops through throttle passages39, 40 are equal. The opposing pressures in reaction chambers are thenequal and the reaction forces on the valves are balanced.

Assume a small movement of control valve shift, e.g., in FIG. 3 pistonvalve 2 goes to the right, or in FIG. 4 piston valve 45 shifts to theright and piston valve 46 goes to the left. Due to the large inlet gaps19, 20 as compared with return gaps 22, 23, a slight shift causes nosignificant change at gaps 19, 20 but at gap 22, 23 there is significantpressure change. However, there is no significant pressure change in thechambers of servomotor 14.

There is an increase of flow via 19-22 and a decrease via 20-23, as willbe apparent from the widening of gap 22 and the narrowing of gap 23.

Thus, the pressure gradiant at the throttle passage 39 increases, whilethe pressure gradient at throttle passage 40 decreases.

Accordingly, the pressures in grooves 8A and 9A have insignificantchange, but the pressures in grooves 8B and 9B have definite change inthe region of the ports to reaction chamber passages 28, 29.

If, as noted above, variable throttle 38 is closed, the difference inpressures at the ports has no effect because, due to check valve 31being closed, no flow can occur from the higher pressure region at 28 tothe lower pressure region at 29. Thus, no flow passes through the fixedthrottle 36, 37 and reaction chambers 24, 25 are pressure equalized.

In this operating area, i.e., variable throttle 38 being closed, therecan be no hydraulic reactive force due to imbalance of pressures in thereaction chambers. Only the mechanical return force of conventionalvalve return springs (U.S. Pat. No. 4,186,818) is active if the steeringwheel is operated and the manual effort cannot be greater than thespring return force Mx₂. This is true even though booster steeringpressure is available.

This condition is illustrated by the characteristic line 61 on FIG. 2, aso-called "cut-off" line illustrating a parking characteristic.

On FIGS. 1 and 2 the differential pressure Δp of the servomotor isploted on the ordinate and the manual steering torque effort Md is alongthe abscissa.

For purposes of steering wheel correction without pressurizing theservomotor while traveling on a straight road at some speed effecting anopen variable throttle 38, only slight steering wheel movement causesvery little piston valve movement.

Thus, a slight movement of piston valve 2 or piston valve 45 to theright and piston valve 46 to the left, return gap 22 opens wider whilereturn gap 23 is narrowed. However, no effective increase or decrease inthe openings of inlet gaps 19 or 20 is occasioned. The widening ofreturn gap 22 permits an increase of flow through throttle passage 39and the narrowing of gap 23 decreases flow through throttle passage 40,causing a greater pressure drop through throttle passage 39 and a lesserpressure drop through throttle passage 40. This difference in pressuresis at the regions to the ports of reaction passages 28, 29 and thus inthe respective reaction chambers 24, 25. Since the inlet gaps 19, 20have had no significant increase, there is no significant increase inpressure flow to the servomotor 14.

However, the differential pressure of the reaction chambers, viz., lowerin reaction chamber 24 and higher in reaction chamber 25 is felt at thesteering wheel as a centering force Hx₂ created by hydraulic reaction,i.e., the vehicle driver feels that the steering wheel is receiving acentering force counteracting inadvertent turning and he correctsaccordingly to eliminate the feeling of such a force.

If now the valves 2 and 45 be further shifted to the right in FIGS. 3and 4, with variable throttle 38 wide open for high speed driving, gaps19 and 23 assumed closed. There is flow from servopump 4 via groove 7,passage 11, line 13, to the right side chamber of servomotor 14 andthence via passage 29, check valve 31 to reaction chamber 25. Returnflow is from the left side of servomotor 14 via passage 10, line 12,throttle 39, gap 22, line 17 to tank 18. There is a pressure drop fromreaction chamber 25 via fixed throttle 37 to tank 18, but no flowthrough fixed throttle 36, gap 23 being closed. Therefore there is nopressure difference and reaction chamber 24 has only the pressure oftank 18. This condition gives the highest reaction force.

Assume now that variable throttle valve 38 is fully closed as it wouldbe for low speed as in parking. The flow to servomotor 14 is still tothe right side chamber and from the left side chamber as above. However,there is now flow via passage 29, check valve 31 to reaction chamber 24from whence it can go no further because variable throttle valve 38 isclosed. Therefore, the pressures are the same in both reaction chambersand there is no net force of steering resistance.

Assume now the same shifted conditions as just preceding, but variableshifted throttle valve 38 is half open, the flow to and from servomotor14 being as described. There is an increased pressure drop over variablethrottle 38, being half open. Accordingly, there is a pressuredifferential in favor of chamber 25 of medium degree, not as great asfor a wide open variable throttle 38. Such steering resistance is formedium speed.

Referring now to FIG. 2, the characteristic line 61 as mentionedrepresents a "cut-off" line, i.e., variable throttle 38 closed forparking at low speed. Line 62 represents high speed, variable throttle38 wide open. Line 63 represents medium speed, variable throttle 63 halfopen. Noting the horizontal portion of line 62, it is longer than thatfor line 61, being the sum of Mx₂, mechanical centering force and Hx₂,hydraulic centering force, viz., reactive force effecting steeringresistance. The contrast with FIG. 1 for the same lines is evident as animprovement over the prior art.

In general, for speeds below high speed, the variable throttle 38 willbe partially closed causing a pressure just upstream of the variablethrottle valve 38, e.g., at junction 34. This results in a reduction ofpressure differential between reaction chambers 24 and 25. Accordingly,the hydraulic centering force decreases as indicated in FIG. 2 by line63 showing a shortened extent of horizontal components.

Considering the discussion of operation above, it will be apparent thatdue to the complete symmetry of the components in FIGS. 3 and 4, if areverse shift of the piston valves is had, the hydraulic reaction in thechambers 24 and 25 and the flow to and from servomotor 14 and the pistondirection will all obviously be reversed.

In further detail of discussion of FIGS. 3 and 4, whenever the variablethrottling valve 38 is closed, only the mechanical spring force (U.S.Pat. No. 4,186,818) is available for valve centering. There is nohydraulic reaction; the manual force (Md) cannot be greater than themechanical spring force Mx2 even though booster steering pressure isavailable.

When variable throttle 38 is open at high speed travel, pressure flowfrom the servopump can enter reaction chambers 24 and 25 via checkvalves 30 and 31 through the fixed throttles 36 and 37, allrespectively, and thence return via the variable throttle 38 to tank 18.If that throttle be opened so wide that at junction 34 for lines 32 and33 the tank pressure prevails, the reaction chambers 24 and 25 have noeffect on each other.

Neglecting flow resistance, e.g., of check valve 30 and 31, and variablethrottle valve 38 being open, the pressure in reaction chambers 24 and25 is the same as at the ports to respective passages 28 and 29. Ifpressure in passage 28 becomes less than at passage 29, the valve meansbeing shifted only slightly the unbalance of pressure forces in thereaction chambers acts against shift of the valve means. However, inletflow control gaps 19 and 20, in a slight shift being open, theservomotor chambers have equal pressure, neglecting as mentioned aboveall unintentioned flow resistance in the system. Accordingly, thehydraulic unbalance is in the nature of a mechanical centering force onthe valve means which is additive to the mechanical spring force.

Further shift of the valve means, e.g., commencing the closing of returnflow through gap 23 results in increased flow through piston valve 2 ofFIG. 3, or through piston valve 45 (FIG. 4). This results in a greaterflow at throttling collar 41, thus resulting in a pressure differentialbuild up in passages 28 and 29 until the shift causes inflow gap 19 tobecome fully effective due to full closure at return gap 23. At thattime the right side chamber of servomotor 14 is pressurized for powerboost.

The operation of FIGS. 5 and 6 is the same as previously described. Thedistinction in FIG. 5 is that the throttling elements on the pistonvalves are axially elongated collars 55, 56 rather than narrow collars41, 42 of FIGS. 3, 4 and 6. In FIG. 6 collars 41, 42 are set off at Gfor free flow in neutral until the collars shift to coact with housinglands to effect throttling gaps 59, 60 much the same as the throttlinggaps 39, 40 of FIG. 4 or 55, 56 of FIG. 5.

FIG. 7 shows a variation of FIG. 6 is structure. In FIGS. 3-6 thethrottling passages 39, 40 (FIGS. 3, 4) or 55, 56 (FIG. 5) or thethrottling passages 59, 60, 41, 42 (FIG. 6), together with therespective throttling collars, are located between the servomotorpassages 10, 11 on one side and the return flow control gaps, 22, 23 onthe other side. In FIG. 7 the throttle collars 64, 65 are set off at Gfor free flow in neutral and throttle passages of respective pistonvalves 66, 67 are located between the servomotor passages 10, 11 and theinlet flow control gaps 19, 20.

Such variation is also usable in the modifications of FIGS. 3-5.

The operation of FIG. 7 is the same as in FIGS. 3-6 insofar as responseto variable throttle valve 38 is concerned. Thus, for a small shift ofvalve means the pressure difference between the reaction chambers 24, 25changes, but the pressure difference in the chambers of the servomotoris insignificant. The mechanical action and gap control sequence is thereverse of that in FIGS. 3-6.

Persons skilled in the art can vary the characteristics shown in FIG. 2as a matter of design and selection of gap dimensions and controlsequence of openings.

In retrospect:

It will be noted that in all modifications the flow to the reactionchambers 24, 25 via passages 26, 28 or 27, 29, respectively, fromrespective servomotor passages 10, 11 must pass through respectiveinternal annular throttle passages in the housing bores of the severalmodifications, e.g., throttle passages 39, 40 of FIGS. 3 and 4; 55, 56of FIG. 5; 59, 60 of FIGS. 6 and 7.

In neutral, FIGS. 3-6 inlet flow control gaps 19 and 20 are wider thanthe respective return flow control gaps 22 and 23. In FIG. 7 gaps 22, 23are wider than respective gaps 19, 20.

In FIGS. 3-6 the return flow control gaps 22, 23 are adjacent and closerto the ports for passages 28, 29 respectively, than are the respectiveinlet flow control gaps 19 and 20. FIG. 7 has an opposite relationship.Accordingly, variations in design are possible.

Further, with regard to the neutral position of FIGS. 6 and 7, the axialoffset of the collars 41, 42 and 64, 65, respectively, for free flow inneutral, effect open gaps smaller than the inlet flow control gaps 19,20 and the outlet flow control gaps 22, 23 to ensure throttling forpressure drop before steering control becomes effective.

Common to all modifications is the locating of all the internal throttlepassages between inlet flow control gaps 19, 20 on the one side andreturn flow control gaps 22, 23 on the other side. In FIGS. 3-6 theinlet flow control gaps 19, 20 are upstream of the respective throttlingpassages, while the return control flow gaps 22, 23 are downstream. Theopposite is true of FIG. 7.

In all modifications a throttling passage (39, 40 etc.) is between aservomotor passage 10, 11 and a respective reaction chamber 24, 25, sothat flow from a servomotor passage to a reaction chamber must alwayspore through a throttling passage to cause a pressure increase in areaction chamber as compared with a pressure decrease in the opposedreaction chamber.

Thus, differential pressures in the reaction chambers are felt assteering resistance forces by the vehicle driver, but no boosterpressure to the servomotor occurs unless the valve means is manuallyshifted to a predetermined degree and provided that the speed responsivethrottle is not closed.

I claim:
 1. In a motor vehicle steering system of the kind described forpressurizing and exhausting chambers of a double actingservomotor;comprising a pair of piston valves (2) (45, 46) (49, 50) (66,67) manually shiftable in bore means (43, 53, 54) in a housing (3) (44)and exposed to opposing pressures in respective reaction chambers (24,25) and having coaction with said housing to provide inlet flow controlgaps (19, 20) and return flow control gaps (22, 23) for pressurizing andexhausting said servomotor chambers wherein said gaps are open inneutral position; means (26, 28) (27, 29) providing flow passage fromsaid servomotor chambers to respective reaction chambers throughrespective check valves (30, 31) wherein each servomotor chamberconnects to said housing intermediate an inlet flow control gap and areturn flow control gap; means comprising fixed throttle means (36, 37)and a speed responsive variable flow throttle (38) effecting pressurecontrol of said reaction chambers; the improvement comprising: throttlepassage means (39, 40) (55, 56) (59, 60) in said housing operativelydisposed to throttle flow from said servomotor chambers to respectivereaction chambers (24, 25) at pressures determined by flow through saidrespective throttle passage means; either of said flow control gap meanscomprising gaps predeterminedly wider in neutral position than the gapsof the other gap means whereby flow through said valve means isdependent on predetermined shifted positions of said valve means toinitially cause equal or unequal opposed pressures in said reactionchambers responsive to closure or opening of said speed responsivethrottle (38) and throttling of flow to respective reaction chambers bysaid throttle passage means; wherein said servomotor is pressurized onlysubsequent to a further predetermined degree of shift of said valvemeans independently of an open or closed condition of said speedresponsive throttle means; whereby reaction chamber centering force onsaid piston valves is variable and dependent on vehicle speed, varyingfrom low at low speeds to high at high speeds.
 2. In a motor vehiclesteering system as set forth in claim 1, said inlet flow control gaps(19, 20) being wider than said return flow control gaps (22, 23).
 3. Ina motor vehicle steering system as set forth in claim 1, said returnflow control gaps (22, 23) being wider than said inlet flow control gaps(19, 20).
 4. In a motor vehicle steering system as set forth in claim1,wherein said throttle passage means comprises collars (41, 42) (51,52) (64, 65) on said piston valves peripherally spaced from said boremeans in said housing to form throttling passages (39, 40) (55, 56) (59,60) through which flow passes from a servomotor chamber to a respectivereaction chamber.
 5. In a motor vehicle steering system as set forth inclaim 4, wherein said housing has grooves (57, 58) in which said collars(41, 42) (64, 65) are disposed when said piston valves are in neutralposition to effect neutral flow gaps (G) by being axially spaced fromrespective portion of said bore means which coact with said collars toeffect said throttle passage means when said piston valves are shiftedwhereby flow past said collars in neutral position is unimpeded bythrottle effect.
 6. In a motor vehicle steering system as set forth inclaim 5, wherein said neutral flow gaps (G) are narrower than the inletor return control flow gaps.